1. Field of the Invention
The present invention relates to a semiconductor device, and more specially, a power semiconductor device using a ribbon bonding.
2. Description of the Related Art
In a power semiconductor device such as an inverter or the like, high current such as several dozen through several hundred Ampere is flown between electrodes of the device. Because of this, a ribbon having large cross-section area is used for connecting the electrodes. FIG. 7 is a side view of a conventional power semiconductor device having a ribbon bonding, generally denoted at 500. The power semiconductor device 500 includes a base board 1. An insulating substrate 2 having a metallic pattern on each side is fixed on the base board 1 with a solder layer 4. A semiconductor element 5 is fixed on the metallic pattern 3. A relay electrode 6 is also formed on the base board 1. The semiconductor element 5 is connected to the relay electrode 6 through a metallic ribbon 57 (JPA 2004-336043).
The ribbon bonding is performed by pressing the metallic ribbon 57 having an approximately rectangular cross-section onto an electrode (not shown) formed on the surface of the semiconductor element 5 with applying ultrasonic vibration. Consequently, plastic flow of the metallic ribbon 57 arises, so that the metallic ribbon 57 is connected to the electrode.
However, in the event that the above-mentioned high current is provided intermittently to the metallic ribbon 57 of the power semiconductor device 500 according to an operation of an inverter, the metallic ribbon 57 is broken away from the electrode of the semiconductor element 5, which causes failure of the semiconductor device 500. Consequently, there arises a problem that there is a limitation to the guaranteed life-time of the semiconductor 500. The coefficient of linear expansion of the semiconductor element 5 made of silicon, for instance, is approximately 2.3 ppm/degree, while the coefficient of linear expansion of the metallic ribbon 57 made of aluminum, for instance, is 23 ppm/degree which is approximately 10 times as much as that of the semiconductor element 5. Therefore, in the event that the semiconductor device 500 having heating value of several hundred W, for instance, operates, the connecting portion between the metallic ribbon 57 and the electrode is heated to the temperature of a hundred and several dozen degree, and then thermal stress is generated at the connecting portion based on the difference of the coefficients of linear expansion. It is considered that the thermal cycle fatigue is generated at the connecting portion by the difference in temperature between the temperature when current is applied and the temperature when no current is applied, which causes the separation of the metallic ribbon 57.
Correspondingly, it is suggested that the connecting strength is increased by increasing the pressure or the oscillation energy applied during the bonding step, or by increasing the amount of time for the bonding step. However, there arises a problem that the bonding machine becomes larger and more complex or a problem that the semiconductor element is destroyed during the bonding step, or the like.
As shown in FIGS. 8A and 8B, the metallic ribbon 57 is deformed to be protruded in the direction parallel to the connecting surface. FIG. 8A shows a cross sectional view of the metallic ribbon 57 used for the power semiconductor device 500, and FIG. 8B shows a cross sectional view of the metallic ribbon 57 connected to the semiconductor element 5. In FIGS. 8A and 8B, the same numerals as those of FIG. 7 denote identical or corresponding parts.
As shown in FIG. 8B, during the ribbon bonding step, the metallic ribbon 57 is deformed in the direction indicated by the arrow 9, and the protruded portion 10 is formed. As the pressure or the oscillation energy applied during the bonding step become larger, the amount of the deformation of the protruded portion 10 also becomes larger. Consequently, there arises another problem that the downsizing of the power semiconductor device 500 is difficult when the strength of the connection is increased by increasing the pressure or the like during the bonding step.